The present invention relates to planar optical waveguides used for optical communication, optical information processing or the like, and to methods for manufacturing the same. In particular, the present invention relates to planar optical waveguides made of polymer material and to methods for manufacturing the same.
As the technology of optical information processing advances and optical communication systems are put into practice, there is a need for the development of a variety of components for optical communications, such as optical transmission lines, semiconductor lasers and photodetectors. Of these, optical transmission lines transmitting optical signals are especially important, and necessary requirements are that the optical losses are small, and manufacturing is easy.
As optical transmission lines, there are quartz-based optical transmission lines, which are based on quartz, and organic polymer optical transmission lines, which are based on an organic polymer material. Among these, even though in optical transmission lines having a planar optical waveguide that is organic polymer based (polymer waveguide), the transparency, heat resistance and performance are poorer than that of quartz-based transmission lines, their flexibility is excellent and they can be easily formed into transparent films, and they are promising with regard to their low number of manufacturing steps, their low cost, etc.
As the material for polymer optical waveguides, many polymer materials, from fluorinating polyimides to polymethylmethacrylate, silicone resins and epoxy resins, have been disclosed, for example in Japanese Laid-Open Publication No. 09-251113, or by Shin Hikida, Saburo Imamura in xe2x80x9cDENSHI ZAIRYOxe2x80x9d, page 32, Feb. 1996 issue, and by Tohru Maruno in xe2x80x9cOYO BUTURIxe2x80x9d, vol. 68, 1st issue (1999) among others.
Referring to FIGS. 8A and 8B, the following is an explanation of a conventional polymer-based planar optical waveguide. FIGS. 8A and 8B schematically show the cross-sectional structure of the planar optical waveguide.
In the conventional planar optical waveguide shown in FIG. 8A, a lower cladding layer 200 having a groove with quadrilateral cross section is formed on a substrate 100, and a core layer 300 made of an organic polymer material is filled into this groove. Furthermore, an upper cladding layer 400 is formed such that it completely covers the core layer 300.
In an alternative conventional planar optical waveguide as shown in FIG. 8B, a lower cladding layer 201 is formed on a substrate 101, a core layer 301 made of an organic material with a quadrilateral cross section is formed on the lower cladding layer 201, and an upper cladding layer 401 is formed such that it completely buries the core layer 301.
If the cross section of the core layers 300 and 301 is quadrilateral as in these conventional optical waveguides, there is the possibility that the optical path length of the light guided by reflection along the optical waveguide becomes longer than necessary. Furthermore, optical transmission losses and distortions occur at the boundaries between the different sides of the quadrilateral. Therefore, it is desirable that the cross-sectional shape of the core layer is circular.
However, in conventional planar optical waveguides, the cross-sectional shape of the core layer is quadrilateral due to manufacturing considerations. In the approach shown in FIG. 8A, the groove of the lower cladding layer 200 is formed by etching, so that the cross-sectional shape of the core layer 300 becomes quadrilateral. Similarly, also in the approach shown in FIG. 8B, the core layer 301 itself is formed by etching, so that the cross-sectional shape of the core layer 301 becomes quadrilateral.
The applicant of the present application has investigated several approaches for making the cross section of the optical waveguide circular, and disclosed them for example in Japanese Patent Application No. 2000-180648. However, in these approaches, a process that is completely different from existing processes is used, which creates the new problem of high costs.
On the other hand, to realize an optical transmission line with low optical losses, a uniform transparency without optical dispersion and high-quality film forming properties are desired for the polymer material of the optical waveguide that serves as the core layer. To realize a core layer with high transparency, bulky molecular components are introduced into the polymer in conventional polymer optical waveguides, in order to prevent inter-molecular stacking and crystallization and to achieve amorphousness. However, this approach causes the problem that the polymer matrix tends to become brittle.
In view of these problems, it is a first object of the present invention to provide a planar optical waveguide having a substantially circular cross section with an easy process and a method for manufacturing the same. It is a second object of the present invention to provide a polymer optical waveguide with more uniform transparency and with excellent durability, adhesiveness, etc.
A first planar optical waveguide in accordance with the present invention comprises a layered film formed on a substrate, and an optical waveguide core formed in the layered film, wherein a cross section of the optical waveguide core is substantially quadrilateral, wherein a dopant layer including refractive index-lowering molecules is provided around the optical waveguide core having a substantially quadrilateral cross section, and wherein the refractive index-lowering molecules included in the dopant layer are unevenly distributed in the optical waveguide core with a concentration that is higher toward outer sides and corners of the optical waveguide core, whereby a graded-index optical waveguide is constituted. It should be noted that in the present specification, xe2x80x9caroundxe2x80x9d does not necessarily mean xe2x80x9cencirclingxe2x80x9d but is mainly used in the sense of xe2x80x9cdisposed to the side ofxe2x80x9d or xe2x80x9cdisposed in the vicinity ofxe2x80x9d.
In a preferred embodiment, the dopant layer is formed on the substrate, and the optical waveguide core is formed on the dopant layer.
In a preferred embodiment, the dopant layer is formed on an upper side of the optical waveguide core.
It is preferable that the optical waveguide core includes a polymer material, the refractive index-lowering molecules include fluorinated compatible molecules whose fluorine concentration is higher than that of the polymer material, and the fluorinated compatible molecules are reacted with reactive groups included in the polymer material to immobilize the fluorinated compatible molecules by chemical bonding.
It is preferable that the polymer material is at least one fluorinated polymer material selected from the group consisting of fluorinated polyimide, fluorinated polysiloxane and fluorinated polymethacrylate resins, and the refractive index-lowering molecules include fluorinated compatible molecules whose fluorine concentration is higher than that of the fluorinated polymer material.
A first method for manufacturing a planar optical waveguide in accordance with the present invention includes (a) a step of forming a first dopant film including refractive index-lowering molecules on a substrate, (b) a step of forming a thin film to serve as optical waveguide core on the substrate, and subsequently forming an optical waveguide core with substantially quadrilateral cross section by etching the thin film, (c) a step of forming a second dopant layer including refractive index-lowering molecules on an upper side of the optical waveguide core with substantially quadrilateral cross section, and (d) a step of doping the refractive index-lowering molecules from the first and second dopant layers into the optical waveguide core with substantially quadrilateral cross section, whereby the refractive index-lowering molecules is distributed unevenly with a concentration that is higher toward outer sides and corners of the optical waveguide core.
In a preferable embodiment, step (d) includes a thermal processing step.
It is preferable that the refractive index-lowering molecules are fluorinated compatible molecules, and that by at least one process selected from the group consisting of UV light processing, electron beam processing, plasma processing and thermal processing, a polymer material constituting the optical waveguide core is reacted with reactive groups included in the fluorinated compatible molecules, which are the refractive index-lowering molecules with which the optical waveguide core is doped, whereby the polymer material and the fluorinated compatible molecules are immobilized by chemical bonding.
A second planar optical waveguide in accordance with the present invention has an optical waveguide core, the optical waveguide core is formed over a substrate, a low refractive index layer including refractive index-lowering molecules is formed around the optical waveguide core, and the optical waveguide core includes the refractive index-lowering molecules at its periphery.
It is preferable that the refractive index-lowering molecules are distributed with higher concentration toward the outer sides of the optical waveguide core.
A second method for manufacturing a planar optical waveguide in accordance with the present invention includes a step of forming a dopant layer including refractive index-lowering molecules on a substrate, and a step of forming an optical waveguide core on the dopant layer, and subsequent thermal processing.
A third method for manufacturing a planar optical waveguide in accordance with the present invention includes a step of forming an optical waveguide core on a substrate, and a step of forming a dopant layer including refractive index-lowering molecules around the optical waveguide core, and subsequent thermal processing.
A fourth method for manufacturing a planar optical waveguide in accordance with the present invention includes a step of forming a first dopant layer including refractive index-lowering molecules on a substrate, a step of forming an optical waveguide core on the first dopant layer, and a step of forming a second dopant layer including refractive index-lowering molecules on the first dopant layer, covering the optical waveguide core, and subsequent thermal processing.
It is preferable that heating is performed such that lines of equal concentration of the refractive index-lowering molecules in a cross section of the optical waveguide core become substantially circular.
A polymer optical waveguide in accordance with the present invention, made of a polymer composition obtained by adding, to at least one fluorinated polymer material selected from the group consisting of fluorinated polyimide, fluorinated polymethacrylate and fluorinated polysiloxane, fluorinated compatible molecules whose fluorine concentration is higher than that of the fluorinated polymer material.
It is preferable that the fluorinated compatible molecules are unevenly distributed with a concentration that is higher toward outer sides of a cross section of the polymer optical waveguide.
In a preferred embodiment, reactive groups included in the fluorinated compatible molecules are reacted with reactive groups included in the fluorinated polymer material to form chemical bonds.
In a preferred embodiment, the fluorinated polymer material is a fluorinated polyimide, and the fluorinated compatible molecules are a fluoride selected from the group consisting of
(1) polyvinylpyrrolidone,
(2) (methylmethacrylatexe2x80x94vinyl pyrrolidone) copolymer, and
(3) composition including polymethylmethacrylate and (methylmethacrylatexe2x80x94vinyl pyrrolidone) copolymer.
It is preferable that the fluorinated polymer material is a fluorinated polymethylmethacrylate resin, and the fluorinated compatible molecules are an organic compound including a tertiary fluoromethyl group.
It is preferable that the organic compound including a tertiary fluoromethyl group includes at least one selected from an OH group, an epoxy group and an isocyanate group, and at least one of the OH group, the epoxy group and the isocyanate group is reacted with a carboxyl group in the fluorinated polymethylmethacrylate resin to form a chemical bond, whereby immobilization is achieved.
It is preferable that the fluorinated polymer material is a fluorinated polysiloxane, and the fluorinated compatible molecules are a siloxane skeleton compound including a tertiary fluoromethyl group.
It is preferable that the siloxane skeleton compound added to the fluorinated polysiloxane has at least one of a Sixe2x80x94OH group and a Sixe2x80x94Cl group, and chemical bonds are formed by reacting the at least one of a Sixe2x80x94OH group and a Sixe2x80x94Cl group with a reactive group in the fluorinated polysiloxane.
It is preferable that an organic compound including the fluorinated compatible group and an incompatible group including active hydrogen is added at not more than 2 wt % to the fluorinated polymer material.
It is preferable that the fluorinated compatible group is at least one selected from the group consisting of xe2x80x94CF1xe2x88x923H2xe2x88x920, xe2x95x90CF2, xe2x80x94CnFmH2nxe2x88x92m+1 (with nxe2x89xa71, 2nxe2x89xa7mxe2x89xa71), xe2x80x94CnFmH2nxe2x88x92m (with nxe2x89xa71, 2nxe2x89xa7mxe2x89xa71), and xe2x80x94C6FmH6xe2x88x92m (with 5xe2x89xa7mxe2x89xa71), and the incompatible group including active hydrogen is at least one selected from the group consisting ofxe2x80x94CONH2, xe2x80x94NH3, xe2x80x94OH, and xe2x80x94COOH.
The planar optical waveguide of the present invention includes refractive index-lowering molecules at a periphery of an optical waveguide core portion formed on a substrate, whereby an optical waveguide having a refractive index difference in the optical waveguide core can be formed, and a graded-index planar optical waveguide can be obtained. Furthermore, in the method for manufacturing a planar optical waveguide of the present invention, a dopant layer including refractive index-lowering molecules is formed on a substrate, an optical waveguide core is formed on this dopant layer, and then thermal processing is performed. When performing thermal processing in this manner, the refractive index-lowering molecules in the dopant layer migrate into the optical waveguide core, so that a substantially circular optical waveguide can be formed in the optical waveguide core.
A polymer optical waveguide in accordance with the present invention is made of a polymer composition obtained by adding, to at least one fluorinated polymer material (fluorinated polymer matrix) selected from fluorinated polyimide, fluorinated polymethacrylate and fluorinated polysiloxane, fluorinated compatible molecules whose fluorine concentration is higher than that of the fluorinated polymer material. With this configuration, fluorinated compatible molecules having a fluorine concentration that is higher than that of the fluorinated polymer material are given to that fluorinated polymer material, and the fluorinated compatible molecules act as a plasticizer in the polymer material (matrix), so that a solid solution is formed, and an amorphous molecular aggregate is formed. Consequently, it is possible to obtain an excellent core layer with uniform transparency and without optical dispersion. Furthermore, by adding, to a fluorinated polymer material, fluorinated compatible molecules having a fluorine concentration that is higher than that of the fluorinated polymer material, the refractive index of the portion to which the fluorinated polymer material has been added can be lowered.
In accordance with the present invention, a dopant layer including refractive index-lowering molecules is provided around an optical waveguide core, and the refractive index-lowering molecules included in the dopant layer are unevenly distributed in the optical waveguide core with a concentration that is higher toward outer sides and corners of said optical waveguide core, so that a planar optical waveguide with substantially circular cross section can be accomplished with a simple process.
Furthermore, a polymer optical waveguide of the present invention is made of a polymer composition obtained by adding, to at least one fluorinated polymer material selected from the group consisting of fluorinated polyimide, fluorinated polymethacrylate and fluorinated polysiloxane, fluorinated compatible molecules whose fluorine concentration is higher than that of the fluorinated polymer material, so that the uniform transparency of the optical waveguide is improved, and an optical waveguide with excellent durability and adhesiveness is obtained.